Prion diseases or transmissible spongiform encephalopathies (TSES) cause progressive degenerative disorders of the central nervous system resulting in death (Prusiner, Med. Res. Rev. 16:487, 1996; Weissman, FEBS Letters 289:3, 1996). Scrapie, a TSE in sheep, was first described over 200 years ago (Pattison, Vet. Rec. 123:661, 1988), and is the prototype of these diseases. There are no known treatments for these diseases and no known antemortem tests for the presence of the disease in an animal. Prion diseases are caused by a conformational change of the normal host prion protein to an abnormal structure that forms aggregates. Because of the recent outbreak of bovine spongiform encephalopathy in the United Kingdom and the connection between this TSE and the new variant, Creutzfeld-Jakob (Bruce et al., Nature 389:498, 1997), a human TSE, there is a need for new methods that are both sensitive and accurate to diagnose TSEs. Ideally, this diagnosis could be used to test animals before they show clinical signs and before they enter the human food chain or into pharmaceuticals prepared for human use.